Honestly, even taking the Fleet Admiral bounty at face value, it's really low comparatively considering how much of a force he is in command of, he controls five top tiers excluding himself (Kizaru, Ryokugyu, Garp, Fujitora, Sengoku), dozens of Vice Admirals, and hundreds of thousands of fodder yet it's lower than Whitebeard's bounty and slightly higher than Kaido's, both of whom have an army that is significantly weaker than the entire Marines.